Move key's headers check outside of eblob_fill_write_control_from_ram

[shaitan]

This check was added in 9685074 and it is intended to determine broken records and incongruity between index and blob.

This check should be moved outside of eblob_fill_write_control_from_ram because eblob_fill_write_control_from_ram is always called under global lock on b->lock and additional seek under this lock leads to degradation of response time.

Our load tests showed follow degradation:

quantile	original v0.23.10	v0.23.10 with reverted 9685074
99%	82.0 ms	26.0 ms (-56.0)
98%	27.0 ms	16.0 ms (-11.0)
95%	26.0 ms	10.0 ms (-16.0)
90%	26.0 ms	7.0 ms (-19.0)
85%	21.0 ms	5.0 ms (-16.0)
80%	18.0 ms	3.0 ms (-15.0)
75%	15.0 ms	3.0 ms (-12.0)
50%	6.0 ms	2.0 ms (-4.0)

In both tests timeout on the client was set to 25 ms.

I will deal with this issue in the near future (1-2 weeks), but until then I'm ready to discuss the proposals where this check can be moved or how it can be transformed.